A group of humans migrating out of Africa some 40,000 to
70,000 years ago mingled with an as-yet unknown branch of humanity, researchers
say.

Modern humans originated about 150,000 to 200,000 years ago in Africa.
However, scientists have long debated when and how the modern human lineagespread out of Africato nearly every corner of the globe. Nearly everyone
outside Africa descended from an exodus that occurred between 40,000 and 70,000
years ago, but recent archaeological findings and climate models suggest that
migrations of modern humans from Africa began at least 100,000 years ago.

One way to find out whether, in the past, modern humans dispersed from
Africa in one wave or many — and to see if they intermingled with any other
human lineages along the way — is to examine the genomes of present-day modern
humans.

“We’re interested in understanding how our species has come
to be how it is through the lens of ancient DNA,” said Swapan
Mallick, bioinformatics director at Harvard Medical School in Boston and lead
author of one of the three studies appearing in the Sept. 22 issue of the
journal Nature.

Previous human genetic databases often sampled a relatively narrow range of
populations, which could skew results or miss key details about the migrationsof modern humans out of Africa. Now, three studies have collected new,
high-quality data from 787 human genomes from more than 280 geographically
diverse populations around the world, including typically understudied and
rapidly disappearing groups.

Among the understudied groups researchers looked at are African
populations, which have considerable genetic, linguistic and cultural
diversity. They also examined genomes from Australia, where previous research
uncovered some of the earliest archaeological and fossil evidence of modern
humans outside Africa.

Recent work suggested that Denisovans have contributed about 5 percent of
their DNA to the genomes of present-day people of the Pacific islands of
Oceania. However, these new findings suggest that what seemed to be evidence of
Denisovans in the Pacific were actually signs of an unknown human lineage.

“These guys were very distantly related to Denisovans, but
by no means Denisovan,” Willerslev told Live Science. “They were even more
distantly related to Neanderthals, and they might have been even more distantly
related to modern humans. We believe that they interbred with modern humans
shortly before modern humans crossed into the ancient continent of Sahul — what
is now Australia, New Guinea and Tasmania — some 50,000 to 60,000 years ago.”

OLDEST LIVING POPULATION

The scientists also discovered that aboriginal Australians “are one of
the oldest living populations on Earth, and have been in the same area for the
past 50,000 to 60,000 years,” Willerslev said.

There was a great deal of controversy “over whether or not aboriginal
Australians directly descend from the first humans entering Australia,”
Willerslev said. “The answer to that question is yes — our data is
completely consistent with aboriginal Australians descending from the first
humans to enter Australia. It shows a very long connection between those people
and the land.

“I can’t think of any other place in the world where humans
have been so long in the same spot as Australia,” Willerslev said. “Yes,
there are populations in Africa that are older, but we have no idea if they
stayed in the same area in Africa for as long a time.”

This is the first comprehensive population-level whole-genome study of
human genetic diversity in Australia. “We found that because aboriginal
Australians have spent such a long time in Australia, they are very genetically
diverse,” Willerslev said. “An aboriginal Australian from eastern
Australia and one from southwestern Australia are almost as different
genetically as an Asian is from a European.”

The researchers noted that about 90 percent of aboriginal Australians speak
languages belonging to a single linguistic family, “but some people in
northwest Australia speak other language families,” Willerslev said. “It’d
be very interesting to see what the story is there when it comes to how they
migrated to Australia.”